Counterpulsation heart assist devices must be controlled to operate in a predetermined time relationship with the pulsing of a patient's heart. For example, the counter-pulsation heart assist devices disclosed in the Applicant's above noted PCT patent application are configured to compress the aorta in synchrony with the diastolic period, the beginning of which is marked by closure of the aortic valve (which produces an audible sound known as second heart sound, or S2) to reduce the interior volume of the aorta during diastole. This compression increases systemic blood pressure, increases blood flow through the coronary arteries and increases diastolic output against the closed aortic valve. The compression of the aorta is alternated with periodic withdrawal of aortic compression following the R wave of the ECG (indicating ventricular depolarisation), around the time, known as presystole, of the closing of the mitral and tricuspid valves (audibly, the first heart sound, or S1) and opening of the aortic valve (marking the beginning of systole) to allow the aorta to return to its normal interior volume. This withdrawal of compression of the aorta at the time the heart is first ejecting blood from the left ventricle unloads the heart so that it can eject blood from the left ventricle more easily. Timing of deflation of the device in relation to the R wave or the detected first heart sound (S1), and inflation in relation to the second heart sound (S2) may be varied according to specific patients' physiology.
It will be apparent to those skilled in the art that the extent of heart assistance provided by counter-pulsation heart assist devices depends upon accurately timing the compression and decompression of the aorta relative to the timing of the native heart. In embodiments of the invention which relate to co-pulsation of the heart, the timing of compression of the heart must be also timed to its native rhythm.
Current devices rely on the ECG, particularly the R wave, to time deflation of the cuff, and inflation of the cuff may be timed to the dichrotic notch of the aortic pressure tracing, which indicates aortic valve closure. This methodology is severely limited in its application for control of heart assist devices and the like for a number of reasons:
1. The ECG is good for indicating the beginning of ventricular contraction, but does not indicate the end of systole. Further, whilst the T-wave indicates ventricular repolarisation, it is broad-based and not very accurate for timing purposes;
2. The systemic arterial blood pressure is very good at highlighting the time of aortic valve closure, but is only useful in a temporary manner, via a percutaneous arterial line, and is not suitable for long-term use;
3. The arterial pressure wave form is coupled to, but delayed, the more further peripherally the arterial pressure is measured from the aortic valve, and may not accurately describe the time of closure of the aortic valve; and
4. As the heart rate varies (particularly if the patient is suddenly exercising, or anxious, or the rhythm is in atrial fibrillation, or depending on the contractile state of the myocardium etc), time of opening and closure of the aortic valve after the R-wave of the ECG may vary significantly, thus whilst the beginning of systole can be relatively safely timed (and balloon deflation initiated), the timing of the beginning of diastole is not possible with ECG alone.
There is no reliable and accurate way to determine particularly the timing of aortic valve closure long-term in manner that allows patients to enjoy a good life-style whilst fitted with the device.
Methods are disclosed in U.S. Pat. Nos. 5,904,666 and 6,042,532, assigned to L. Vad Technology, Inc., to transduce the aortic pressure wave form every two to three minutes by taking a measurement of the dichrotic notch of the aortic blood pressure tracing. However, this requires the device functionality to be paused every 2-3 minutes to male measurements. This does not allow precise control of device function to specific heart beats, rather, timings are set for 2 minutes, until the measurement is re-done. Further, the dichrotic notch may not always be detected.
Another problem associated with components used to control partially implantable heart assist devices (i.e. having external drivers/controllers) is that the size, number, and rigidity of any percutaneous tubing or wires must be kept to a minimum to reduce the chance of infection and increase psychological acceptance of the devices. This can be achieved by the use of wireless transmission of cardiac cycle timing signals. However, the wireless telemetry associated with pacemakers is usually proprietary and unnecessarily complex, and is not suited for continuous discreet signal outputs
It is an object of the present invention to provide methods and devices for determining and adjusting counterpulsation inflation timing by using detected heart sounds. In preferred embodiments, the heart sounds are monitored real-time to cause accurate beat-to-beat counterpulsation timing for each specific cardiac cycle, without interruption of heart assist functionality. Heart sounds may also be used intermittently to determine and reset the interval between R wave balloon deflation or inflation, either at fixed time intervals, or when there is a sustained change in the heart rate.
Another object is to provide, again at least in preferred embodiments, simple and economical wireless telemetry of the detected signals to an external device.